Erect or scandent, mostly glabrous herbaceous or subshrubby perennials with persistent, evergreen or annually withering branches growing from a compact or creeping, sympodial rhizome or rarely a tuber (A. Ovatum and A. undulatum); cataphylls small or vestigial. Roots cylindrical or fusiform, lateral roots sometimes tuberous, covered by a multiple velamen. Stems erect or voluble, their branches often patent; leaves on long shoots and short shoots (if the latter present) scalelike, scariose, spurred at the base and subtending solitary or clustered phylloclades, the latter leaflike to angled or terete, rarely (A. densiflorus) bearing minute, reduced scale leaves; spines present or absent, formed from modified leaf spurs or branches. Flowers in fascicles or borne singly, bisexual, or unisexual and monoecious or dioecious, pendulous or erect, on articulated pedicels, the pericladium separated from pedicel by a swelling; tepals 6, free and spreading or basally fused and then forming a cup or tube; stamens 6, free from each other, fused to tepals, or sometimes outer stamens basally adnate, diminute and nonfunctional in pistillate flowers; filaments free, filamentous or flattened; anthers bithecate, sagittate, ± basifixed, introrse, dehiscing by slits; ovary superior, trilocular, sometimes slightly stipitate; style with 3 short stigmatic branches or a capitate or lobate stigma of the Dry or Wet type; ovules axile, 2-12 per locule, hemianatropous or almost atropous, bitegmic, crassinucellate; septal nectaries present; fruit a berry, rarely a nut, red, blue, or black or brown; seed coat black; endosperm containing aleurone, fat and hemicellulose but no starch; embryo slightly curved, nearly reaching the length of endosperm.

VEGETATIVE MORPHOLOGY. Shoot architecture of Asparagus is highly complicated. According to Braun (1849), in the 1st year A. officinalis develops 3-4 shoot generations from a rhizome, followed by 5-6 generations in the 2nd and 8-10 in each subsequent year, which gradually become stronger and branch more extensively. From the 3rd to about the 15th year, the plants are fully developed, but later become weaker. Flowers are formed only from the 3rd year onward. Flat phylloclades are bilateral and anatomically rather leaflike, while the needlelike phylloclades are radial throughout; both types are supplied by several or a single vascular bundle (Reinke 1898). The interpretation of the phylloclades has led to endless debates among plant morphologists (Arber 1924, 1935; CooneySovetts and Sattler 1986) fostered by pecularities such as the inverted vascular bundles of bilateral symmetric phylloclades, but from their axial position it is clear that they are caulomatic. It is small wonder that in their ontogeny some traits characteristic of developing leaves are manifest. Wenck (1935) recognised that the fascicles of phylloclades in the axil of a bract leaf are cincinni with extremely condensed axes. The flowers form part of these shoot systems as basal (first) lateral branches, and the median shoot is capable of further development. There are 3 different kinds of spine: the indurated spur of a modified leaf, as shown in A. albus; indurated pungent-pointed phylloclades, as shown in A. horridus; and short, spinelike, sterile, real branches, as shown in A. stipularis.

VEGETATIVE ANATOMY. Stomata are anomocytic. Vessels with scalariform perforation plates are present in both roots and stems; vessels with simple perforation plates also occur in the roots (Cheadle and Kosakai 1971). Raphide cells are widely distributed. Hairs are mostly lacking. Cooney-Sovetts and Sattler (1986) showed that in A. setaceus the phylloclade is radially symmetrical throughout, with only 1 vascular bundle, whereas in A. densiflorus the phylloclade is dorsiventral with a bilateral portion distally, and 1 or 2 vascular bundles.

FLOWER STRUCTURE. Asparagus is a large and varied genus, including many species with bisexual flowers, and some monoecious or dioecious species (e.g. A. officinalis). Breitenbach (1878) observed that in the same species, individuals with pistillate flowers always have rudimentary staminodes, while individuals with staminate flowers exhibit much variation in the reduction of the pistil, ranging from fully functional to rudimentary. This was corroborated by Galli et al. (1993), who also reported a high incidence of inbreeding depression from andromonoecious plants. These observations are consistent with the hypothesis of the origin of dioecy in Asparagus from hermaphroditism via gynodioecy. The ovary is superior and sometimes slightly stipitate. It has septal nectaries, and 2 or more ovules per carpel. Lazarte and Palser (1979) studied the anatomy and vasculature of the flowers of A. officinalis. The tapetum is secretory and anther wall formation is of the monocotyledonous type (Venkataswarlu and Raju 1958; Lazarte and Palser 1979).

EMBRYOLOGY. Microsporogenesis is successive, and a mixture of isobilateral, T-shaped, and decussate tetrads are formed. Pollen grains are dispersed in the 2-celled state and contain starch (Venkataswarlu and Raju 1958). Ovules are crassinucellate. Embryo-sac development is according to the Polygonum type (Lazarte and Palser 1979). The mature embryo sac is asymmetrical, and this asymmetry (described by Venkateswarlu and Raju, 1958 as a diverticulum or caecum) is exaggerated during seed development. There is a persistent nucellar epidermis of enlarged cytoplasm-rich cells, as also in Hemiphylacus (Rudall et al. 1997a).

FRUIT AND SEED. The fruit is a globose, red, blue or black berry coloured by carotenoids, enclosing 1-several seeds. Seed shape varies from globose to angular, partly depending on the number of seeds in the capsules; seed colour is deep black. Huber (1969) found the outer epidermis of the testallayer much larger than and clearly differentiated from the collapsed inner seed coat tissue. According to his description, the anticlinal walls of the epidermal cells become thicker towards the periphery, so that the melanin masses filling their lumina adopt the shape of truncated cones with hexagonal outlines, particularly well visible when the outer periclinal cell walls are detached. Malcomber and Sebsebe (1993), in contrast, emphasised the frequent presence of two well-developed cell layers, of which the outer, the epidermis, in many species is detached when the seed is removed from the berry, while it is the subepidermic cell layer that carries phytomelan. Robbins and Borthwick (1925) followed up the seed development of A. officinalis and their illustrations seem to support

Huber's view, but there may be much variation in this large genus. The endosperm stores aleurone, lipids and reserve cellulose, but no starch. The cell walls of the endosperm are distinctly pitted.

Habitat

Distribution

The genus is represented in South and tropical Africa, Madagascar and the Mascarenes, in Macaronesia, the Mediterranean region, particularly the eastern part, in central and E Asia and with a single species in Australia.

Uses

Asparagus officinalis, garden asparagus, has been cultivated since ancient Greek times and today is widely spread and economically important. Useful variants can be maintained by vegetative propagation. In the Mediterranean region, all wild species are collected as vegetable, especially in the Easter week. A. albus (white asparagus) is a crop of N Africa. A. setaceus (Kunth) Jessop (=A. plumosus), the asparagus fern, and other species are much used by florists.

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WCSP 2014. 'World Checklist of Selected Plant Families. Facilitated by the Royal Botanic Gardens, Kew. Published on the Internet; http://apps.kew.org/wcsp/ Retrieved 2011 onwards

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